1. Technical Field
The present disclosure generally relates to the field of digital communication and, more particularly, to a time-division-duplex (TDD) orthogonal frequency-division multiplexing (OFDM) communication system.
2. Description of the Related Art
In a modern point-to-point TDD OFDM communications system, such as a G.fast system that is undergoing standardization process at the International Telecommunication Union (ITU) Telecommunication Standardization Sector (ITU-T), a time domain signal consists of a combination of upstream symbols and downstream symbols. The switching time between upstream and downstream symbols should be minimized or unnecessary data bandwidth would be wasted. The basic transmission unit in G.fast is called a “frame”. A frame consists of a series of symbols in one transmit direction (e.g., from point A to point B) followed by a series of symbols in the other transmit direction (e.g., from point B to point A). If the transmission is from FTU-O (i.e., central office of a service provider) to FTU-R (i.e., customer premises) this is referred to as the downstream direction. If the transmission is from FTU-R to FTU-O this is referred to as the upstream direction.
As described herein, the term “symbol position” is used to represent a specific symbol in an upstream or a downstream transmission. For example, a symbol position of 5 in a downstream transmission is the 5th symbol from the beginning of the downstream transmission.
For purposes such as retransmission acknowledgment or fast on-line reconfiguration to accommodate a rapid noise change experienced on the communication channel, it is desirable to have a dedicated management channel separate from the payload (DTU carrying) channel. The name of this management channel is called the RMC (robust management channel) herein. The specific management channel symbol is called a RMC symbol. The naming of this management channel and corresponding symbol thereof is irrelevant to the inventive concept of the present disclosure, although the term “RMC” is used herein to denote a management channel having the functionality described herein. The RMC is transmitted at a fixed symbol position in both upstream and downstream directions and positioned within one symbol for each direction, so there is one RMC symbol upstream and one RMC symbol downstream in every frame.
A RMC symbol can be a unique symbol or shares a symbol with payload data. The content of the RMC symbol could be protected by high-redundancy (e.g., low coding rate) forward error corrections (FECs) and/or high signal-to-noise ratio (SNR) margins since the RMC symbol carries critical operation and management information. It could also use a known bit-loading pattern that does not require any handshake between the FTU-R and FTU-O devices in showtime. Showtime refers to an operation state in which DTUs are transmitted.
One major function of the RMC symbol is carrying the acknowledgement (ACK) information for the retransmission mechanism, similar to the retransmission-return-channel (RRC) defined in G.998.4. Typically in frame-based systems, management data is inserted at the beginning of transmission for each direction to align with the physical layer. However, doing so might affect the performance of the system if the processing time in the digital domain occupies a period of time for multiple symbols. An example is ITU-T G.9960 standard where the time gap between the transmission and the corresponding acknowledgement is defined as IFG (inter-frame-gap). In G.9960 a typical value of inter-frame gap (IFG) is around a time period equivalent to three to six symbols.